1. Field of the Invention
The invention relates to a flat cable for transmitting high bit rate signals comprising at least one pair of conductive wires.
2. Description of the Prior Art
Flat cables used in information technology in the form of a ribbon made of plastics material in which a number of parallel conductors are encased are already known. These cables are produced by extruding the plastics casing material simultaneously around all the conductors. Sometimes the plastics casing material is applied to conductors that are already insulated. It is observed that the conductors are not centered properly, because it is very difficult to achieve a precise positioning of all the conductors in such an extruded ribbon. Consequently, these cables can be used only over a short length, not exceeding a few tens of centimeters, and at relatively low frequencies, in general in the order of a few kilohertz but practically always less than 10 MHz. However, it is preferable to use these flat cables when their use is possible, because the regular disposition of the conductors enables the use of simple insulation displacement connectors which can be fitted very quickly.
In addition, U.S. Pat. No. 3,663,739 describes a flat cable in which the position of the conductive wires with respect to each other can be better. According to this document, a layer is made with ribs that delimit grooves. The surfaces of the grooves and of the layers are covered with a conductive grounding member and insulated conductive wires are seated in the grooves with one conductive wire in each groove. The grooves are then closed, preferably by an additional conductive member. Thus, each conductive wire can be considered as separately shielded.
Such a flat cable which can be used with a simple insulation displacement connector, nevertheless has a drawback. Although the shielding of the separate conductive wires enables crosstalk to be reduced through the electromagnetic protection obtained, there is a capacitive effect between the conductors when they are poorly centered in their insulation.
This drawback is described in detail because it limits the frequencies at which the flat cable can be used. In this flat cable, there is capacitance between each conductor and the ground coating formed over the grooves and the ribs. If all the conductors were truly equidistant from this ground plane, there would be no unbalancing capacitive effect between the conductors. On the other hand, since the conductors are always locally more or less off center in their own insulation, their capacitive effects differ from one conductor to another with respect to a ground plane. A crosstalk phenomenon then appears.
The use of twisted cables with several pairs of conductive wires for the implementation of local area networks is also known to the art. These cables avoid crosstalk the main cause of which is capacitive unbalance between the conductors of different pairs, the conductors and ground. Capacitive unbalance is reduced by the effect of the twists which provide a symmetry. However, these twisted cables have a very important drawback because they do not allow the use of insulation displacement connectors, so that the costs of connecting and laying these cables are of the same order of magnitude as the costs of the cables and their connectors. In addition, laying these twisted cables along partition walls is unsightly because they are very thick.